Here's a place to start: distortion may reduce numerical accuracy because numerical approximations are no longer centered (or symmetric) about the centroid of the volume element. (from http://www.flow3d.com/cfd101/cfd101_grid.html)

no matter what discretisation scheme is used, you end up with a matrix that has to be solved.
see what causes the solution of a matrix difficult/inaccurate then trace it back to the distorted mesh elements